1. Field of the Invention:
This invention relates to a method of forming a fine pattern of ferroelectric film in the production of ferroelectric memories, ferroelectric sensors, superconductive devices, etc., and also to a pattern forming agent suitable for use with such a formation method.
2. Description of the Related Art:
There is currently proposed, as a new semiconductor memory device, a film-like ferroelectric memory which uses a material having a polarization inverting characteristic and being fusible in a semiconductor process. Since it is involatile, such a ferroelectric memory may be used as an involatile memory and requires only a small amount of voltage or current during reloading, thus realizing a practical involatile memory. In an ordinary semiconductor memory such as EPROM, since it is necessary to apply a more excessive current or voltage than usual or to place the memory in an unusable condition in order to clear the stored contents, a separate current source or voltage source dedicated for clearing is needed. However in the proposed ferroelectric memory, since the stored contents can be altered or cleared under normal conditions, it would be unnecessary to equip such a separate current source or voltage source.
The ferroelectric material is widely used, in addition to the above-described ferroelectric memories, in various ferroelectric sensors and liquid crystal displays as well as superconductive devices and functional glass. In these applications, since a ferromagnetic film must be processed into a predetermined pattern, a method of forming a fine pattern of ferroelectric film is important.
In conventional methods, a predetermined pattern is formed using the following process. A ferroelectric film is formed by RF sputtering, whereupon the ferroelectric film is crystallized by fireing. The resulting ferroelectric film is then processed by a fine-pattern treatment, such as chemical wet etching, ion milling such as physical dry etching, or plasma etching using hydrogen chloride or tetrafluoromethane, for example. In wet etching, if the etching solution is an alkaline solution such as of potassium hydroxide or sodium hydroxide, an alkali-proof resist for the photosensitive film is formed by lithography, whereupon film portions other than a predetermined pattern are removed using the etching solution. Thus a desired pattern of ferroelectric film is obtained. If the etching solution is an acidic solution such as of phosphoric acid, sulfuric acid, hydrocloric acid or fluoric acid, an acid-proof resist for the photosensitive film should be used.
However these conventional processes have the following problems:
(1) In wet etching, since it is an isotopic etching, the pattern to be left over tends to become too narrow and/or the resist tends to peel off, and so that fine patterning cannot be achieved.
(2) Though it is excellent for fine processing, ion milling step still has problems such as causing damage to the substrate and making it difficult to select a suitable resist.
(3) Plasma etching has a very poor etching rate, is not suitable for mass production and causes a lot of damage to the substrate, often breaking the active wiring layer under the resist.